Your answer is right there in the article:
“Of course, if the investigation reveals that there should be a criminal prosecution, then that prosecution would take precedence over any report,”
They can’t know if all those subject to prosecution are dead, until they finish the investigation.
The Prosecution side of things seals up things for various reasons.
If Lanza is a lone wolf and they aren’t trying to cover something up then there should be no reason to extend the seal on the search warrants.
http://www.ctpost.com/policereports/article/Sandy-Hook-affidavits-remain-sealed-4150259.php
So what are they hiding
“Sedensky said in his applications that the affidavits contained information “not known to the general public” and that premature disclosure would “seriously jeopardize the outcome and success of the investigation” by “divulging sensitive and confidential information” known only to investigators.”